The present invention relates to a display device such as a plasma display (PD) device. More particularly, the present invention relates to a display device in which the display luminance is determined by the number of light emissions and the total number of light emissions in each cell of a display frame can be altered.
Recently, a display device has been required to be thinner with a larger screen size and to have a finer resolution, plus being capable of coping with the diversification of information to be displayed and conditions under which the product is installed. Types of a thin display devices include types such as LCD, fluorescent display tube, EL and PDP (plasma display panel). In a fluorescent display tube, EL or PDP, a gradation display is generally obtained by composing a display frame of plural subframes, weighting each subframe period in order to be different from each other, and expressing each bit of the gradation data by the corresponding subframe. A description is given below with a PDP as an example. As the PDP device is widely known, a detailed description of a PDP device itself is not given here but only a general description of a general PDP device is given.
FIG. 1 is a block diagram that shows the general configuration of a general PDP device. On a panel 10, plural X electrodes and Y electrodes are arranged adjacently by turns and plural address electrodes are arranged in the direction perpendicular to them. The plural X electrodes are connected commonly and an identical drive signal is applied thereto by an X side common driver 11. The plural Y electrodes are connected to a Y side scan driver 12 individually, and a scan pulse is applied sequentially during an address period. To the Y side scan driver 12, a Y side common driver 13 is connected and an identical drive signal is applied to the Y electrode during a reset period and sustain discharge period. The address electrode is connected to an address driver 14, an address pulse is applied during the address period in synchronization with the scan pulse, and the display cell in the line selected by the scan pulse is selected to be lit or not. A control section 15 internally comprises a display data control section 16, a scan driver control section 17 and a display/power control section 18, and a vertical synchronization signal Vsync, a dot clock and display data are supplied from the outside. The control section 15 comprises a CPU and each part mentioned above can be realized by hardware and by software run by the CPU. To the address driver 14, address pulse data is supplied from the display data control section 16. The X side common driver 11, the Y side scan driver 12 and the Y side common driver 13 are controlled by a scan driver control section 17.
As the method for driving a PDP device, the gradated display by the subframe method and power control have been disclosed, as in Japanese Unexamined Patent Publication (Kokai) No. 2002-99242, no basic description is given here.
As only two-value states are allowed, that is, a state of being lit and a state of being unlit, gradation is expressed by varying the numbers of light emissions in a PDP device. Therefore, the subframe method is employed, in which a frame is divided into plural subframes and the subframes to be lit are combined for display. The number of light emissions (the number of sustain pulses) in each subframe is adequately determined in advance and the maximum number of light emissions in each display cell is the total number of light emissions of all the subframes. As the maximum number of light emissions in each display cell is generally called the sustain frequency, the term is also used here.
When a bright picture is displayed, the total number of light emission pulses in a display frame is increased and the power consumption, that is, the consumed current is increased. The number of light emission pulses in a display frame over the entire screen becomes a maximum when all the cells are lit at the sustain frequency. The display load ratio is used as an index that shows the level of brightness of the entire picture. The display load ratio is a ratio of the total number of light emission pulses in all the cells in a display frame to the maximum number of light emission pulses. The display load ratio is 0% when all the cells are displayed in black, and 100%, when all the cells are displayed at the maximum luminance.
As the current that flows during the sustain period stand predominant as regards the consumed current, if the number of light emission pulses in a display frame is increased, the consumed current is also increased. If the number of sustain pulses in each subframe is assumed to be constant, that is, the sustain frequency is constant, the power consumption P (or consumed current) increases as the display load ratio increases.
A limit is set to the power consumption in the PDP device. It is possible to set the sustain frequency so that the power consumption is below the limit even when the display load ratio is maximum, that is, all the cells are displayed at the maximum luminance. However, the display load ratio of a normal picture is in the range from about 10% to about 30%, so it is highly unlikely that the display load ratio approaches 100% and, as a result, a problem occurs that a normal display becomes dark. Therefore, the power control is taken so that a display as bright as possible is obtained in the range in which the power consumption P is below the limit, by varying the sustain frequency according to the display load ratio. This power control is taken by the display/power control section 18 shown in FIG. 1. Conventional power controls have been disclosed in, for example, the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 2002-99242.
In the plasma display (PDP) device, heat is produced by light emission and discharge in each cell and the amount of produced heat is in proportion to the number of light emissions in a unit of time. Due to this, a large amount of heat is produced locally depending on the display pattern, the distribution of temperature appears on the panel surface, and thermal destruction may be caused at portions where the temperature gradient is large.
In order to solve these problems, the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 2002-99242 has disclosed a technology that can reduce the sustain frequency when a state in which the sustain frequency is large continues and there is the possibility that a thermal destruction will occur, the technology being developed by focusing on the fact that such a problem occurs only when the sustain frequency is large in the case where the sustain frequency is controlled according to the display load ratio.
One of the patterns that cause thermal destruction is, for example, a still picture with high contrast. If such a pattern is displayed for a long time, the phosphors and the like at the pattern are deteriorated and a phenomenon called the burn-in occurs, even if the thermal destruction is not caused. The technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-99242 is simple, but a problem occurs that the brightness is lowered due to the reduction in the sustain frequency even when there is no problem of thermal destruction or burn-in in the case of video.
Japanese Unexamined Patent Publishing (Kokai) No. 10-207423 and Japanese Unexamined Patent Publishing (Kokai) No. 2000-10522 have disclosed a configuration in which control is done so that the luminance is lowered when a display pattern that will cause thermal destruction or burn-in is detected by comparing the display data in successive frames.